(1) Field of the Invention
This invention relates to a device for polishing flat surfaces of metallic parts or ceramic substrates and so forth.
(2) Description of the Prior Art
In order to polish flat surfaces of metallic parts and so forth, a lapping method is generally used in which "corundum" or "Carborundum" works as free abrasive grains, another generally-used lapping method is one in which the above-described abrasive grains are fixed by way of embedding or the like, and a still further method is generally employed in which a surface abrasive disc is used.
However, in a lapping method in which the free abrasive grains are used, a multiplicity of workpieces are treated for a long time in a batch manner. Therefore, an in-line working method in which only one workpiece is individually polished successively cannot be employed. As a result of this, it is difficult to treat the workpiece in a short-time with dimensional accuracy and with consistent finishing performance. In this method, with a multiplicity of working pieces held between the lapping discs by means of a holder, they are simultaneously polished in a batch manner for a long time. Therefore, a great stock during machining or partially fabricated items between manufacturing processes need to be kept, thereby the inventory increases. Furthermore, since a great quantity of fine-abrasive grains is used, running costs become high, and dust is generated causing the work environment to deteriorate. As a result of this, it is difficult to establish a line system in which precision instruments are synchronized with the polishing device. Furthermore, the lapping discs becomes eccentrically worn, and modification of the same become complicated, thereby it takes a long time, as a result of which, an operation rate decreases, causing the running cost to. Furthermore, since the lapping fluid used for machining is difficult to be stored, it needs to be adjusted at short intervals. Furthermore, in this method, manufacturing capacity is too small, and two or more processes such as a rough lapping process and a fine lapping process need to be conducted, therefore, the whole body of the machining device becomes large and expensive. On the other hand, the stationary-abrasive grains lapping method has no significant difference from the above-described method because this method merely employs a lapping disc which contains abrasive grains in place of the lapping disc used in the former method. The abrasive grains fixed by embedding or the like can be easily worn, and a great degree of loading of them occurs. Furthermore, if a thin filmy grindstone like a sandpaper is used, it wears out in a short period of time, therefore, it must be frequently changes. As a result of this, running costs increase significantly, the modification of the filmy grindstone is very difficult, and the manufacture capacity is not significantly different from that conducted by the free abrasive grains. In a method in which a flat grinding disc is used, since both the workpiece and the grindstone are strongly secured, the accuracy of the machine corresponds to the accuracy of the finished surface of the workpiece. Therefore, the cost of the machine becomes high. Furthermore, the roughness of the finished surface which can be achieved by the lapping method is difficult to be achieved in this method. In a grinding method in which a disc-type grindstone is used, if a machining allowance is made large for the purpose of grinding a workpiece in a short time, the grindstone is excessively worn, and good accuracy of the finished surface is not obtained.
Meanwhile, a known method of supporting the workpiece metallic parts or the like is such as a method in which a workpiece is brought into abutment under a certain proper pressure against the top surface of the grindstone which is being rotated horizontally by means of a holding plate secured to the lower end of a vertically-movable shaft. Another method is known in which a grindstone is secured to the lower end of the vertically-movable shaft, while a workpiece is located on a table beneath the grindstone. The workpiece is ground by rotating and lowering the vertically-movable shaft.
However, in such methods, if accurate parallelism is not achieved between the holding plate at the lower end of the vertically-movable shaft and the grindstone disposed beneath the holding plate, or between the grindstone at the lower end of the vertically-movable shaft and the table disposed beneath the grindstone, chatters of the workpiece occur during grinding, and offset wear is generated, thereby an accurate flat plane cannot be obtained, and the roughness deteriorates. As a result of this, stable dimensional accuracy cannot be obtained. Furthermore, the grindstone and the table also create offset abrasion which is very difficult and takes a long time to correct. These problems become further significant if the machining tolerance is enlarged and high pressures are applied to both the grindstone and workpice for the purpose of conducting a short-time machining. In a case where the workpiece is a special shape having a certain roughness on the reverse side thereof which is the other side to the machined side, a proper elastic pad is interposed between this reverse side and the holding plate or the table for the purpose of preventing slippage the workpiece during machining. However, it cannot assuredly prevent the slippage, therefore, it cannot be used for the high pressure machining.
Meanwhile, in order to control the grinding work, a machining condition such as the rotational speed of a grindstone and workpiece or the pressure applied is changed.
However, in order to change the above-described machining conditions during grinding work, a large sized device is needed, also causing costs to rise. Furthermore, good surface roughness in proportion to the costs cannot be obtained, and the most suitable quality range of the grindstone is too narrow, and is impossible to be changed. Especially, the change of the applied-pressure is an important factor in the grinding work, for example, if the applied pressure is high, a grinding force becomes large, causing the time period required to complete the grinding to become short, however the roughness of the finished surface is not sufficient, and chatter of the workpiece will be generated during machining work, deteriorating the grade of the finished surface. On the other hand, if the applied-pressure is low, the grinding force becomes small, and, a fine finished surface can be obtained. That is, by way of including the setting and control of the machining pressure in the grinding work, good surface roughness can be efficiently obtained in a short time. Such change of machining pressure is partially employed by a duplex head flat milling machine in which a predetermined load is first applied by means of an air pressure or the same is applied directly by means of a hydraulic pressure cylinder. In this method, it is difficult to optionally set and control the machining pressure during machining, furthermore, the machining pressure per unit area of a workpiece cannot be increased.